Vehicles travel north from San Diego to Los Angeles along Interstate Highway 5 in California December 10, 2013. (REUTERS/Mike Blake)

A federal judge in Missouri ruled this week held that drivers have a First Amendment right to flash their headlights to warn other motorists of nearby police and speed traps.

The order by U.S. District Judge Henry E. Autrey in St. Louis on Monday stems from a lawsuit filed by Ellisville resident Michael Elli. In 2012, Elli flashed his headlights to warn oncoming vehicles of a radar set up by police in the town of Ellisville.

A flash of headlights is a common way motorists communicate to oncoming drivers of either a dangerous situation or the presence of police — in essence, a warning to slow down.

An officer saw the flash and pulled over Elli, who could have faced a fine of up to $1,000 if convicted. Elli, was accused of "[f]lashing lights on certain vehicles . . . warning of RADAR ahead," according to court papers obtained by The Wall Street Journal.

He faced a fine up to $1,000 in addition to points on his license, according to the report.

The city later dropped the charge, but the American Civil Liberties Union sued on Elli's behalf anyway, claiming the arrest violated his First Amendment right to free speech.

Ellisville City Attorney George Restovich said the city changed the policy after the case went to court and no longer pulls over people for flashing headlights.

"The reality is that the injunction doesn't change the way the city has been operating for the past 12 months," Restovich said.

At a hearing on the lawsuit last year, Ellisville officials made the case that flashing headlights could interfere with a police investigation. But Autrey said in his ruling that the flashing of headlights "sends a message to bring one's driving in conformity with the law — whether it be by slowing down, turning on one's own headlamps at dusk or in the rain, or proceeding with caution."

“The chilling effect of Ellisville’s policy and custom of having its police officers pull over, detain, and cite individuals who are perceived as having communicated to oncoming traffic by flashing their headlamps and then prosecuting and imposing fines upon those individuals remains, regardless” of the city’s decision to change its policy, the judge wrote, according to The Wall Street Journal.

Tony Rothert, legal director of the ACLU’s Missouri chapter told the Journal's Law Blog that the judge's ruling is a civil rights victory for motorists.

"When someone is communicating in a public street, [he is] expressing [himself] in a way that’s protected by the First Amendment,” Rothert said. "Unless there is a strong reason why the government should be allowed to censor that speech, the police shouldn’t be stopping or prosecuting people because of the content of their speech."